This invention relates to electronic timing systems and more especially but not exclusively it relates to systems for the measurement of precise time intervals between events at mutually spaced locations as may for example be required to measure the height of an aircraft by noting the time of reception, at a plurality of spaced apart locations, of a signal transmitted from the aircraft.
A system of the kind just before referred to forms the subject of our co-pending Patent Application GB 0003486.8 to which attention is hereby directed.
The specification accompanying our co-pending Patent Application describes a time offset measurement system for the timing of events at least two spaced apart locations comprising at each location, a dual frequency Global Positioning System (GPS) receiver having operatively associated with it a GPS antenna, wherein both the GPS receiver and the antenna are dual frequency (L1, L2), and the former is capable of both Coarse Acquisition (C/A) Code and carrier phase measurements, a frequency reference source giving local time, a data-logger for logging GPS data, a Time Interval Counter (TIC) used to measure the time of a local event as defined by the frequency reference source relative to time as defined in dependence on GPS data and further comprising a central processor system (CPS), and a communication system via which the logged GPS data and time interval data, are received at the CPS from each location, the CPS being arranged to derive a time-offset figure, in accordance with a predetermined algorithm, which time-offset figure is indicative of the difference between the times measured by the local clocks at the two sites, the time-offset figure thus derived being applied to the time interval measurements to calculate the precise, relative, time difference between events occurring at the two sites.
As described in the specification accompanying the co-pending Patent Application, the system may be used in applications where the timing of events at several widely spaced locations is required, appropriate time intervals being computed in each case. Although for most applications the reliability of the system described is perfectly acceptable, for some applications it is highly desirable that event timing is effected with a very high degree of confidence in timing reliability, so that the development of a system fault which affects timing can be identified without delay.
It is therefore an important object of the present invention to provide system in which this high level of confidence is provided.
According to the present invention, a time offset measurement system for the timing of events at spaced apart locations comprises at each location, a dual frequency Global Positioning System (GPS) receiver having operatively associated with it a GPS antenna, wherein both the GPS receiver and the antenna are dual frequency (L1, L2), and the former is capable of both Coarse Acquisition (C/A) Code and carrier phase measurements, a frequency reference source giving local time, a data-logger for logging GPS data, a Time Interval Counter (TIC) used to measure the time of a local event as defined by the frequency reference source relative to time as defined in dependence on GPS data and further comprising a central processor system (CPS), and a communication system via which the logged GPS data and time interval data, are received at the CPS from each location, the CPS being arranged to derive in accordance with a predetermined algorithm, for each pair of sites a primary time-offset figure based on a direct signal path between the sites, and a secondary time-offset figure based on at least one indirect signal path between the sites, wherein confidence that correct system operation obtains is indicated in dependence upon correspondence between the primary and secondary time-offset figures.
The secondary time-offset figure may be an aggregate time-offset figure based on a plurality of indirect signal paths between the sites.
It will be appreciated when the system is functioning correctly, the primary and secondary time-offset figures for a given pair of sites should be the substantially same (allowing for measurement tolerances) and thus the absence of correspondence therebetween can be deemed to indicate a system fault.